After a film is formed on a substrate, there is a case where a portion of the film is desired to be removed to thin the film. For example, after a sidewall film is formed on a side face of a stacked body on the substrate, there is a case where a portion of the sidewall film is removed to reduce a thickness of the sidewall film. In this case, the problem is how such reduction of the thickness is implemented when the thickness is desired to be reduced by a small amount with high accuracy, for example.